Sapphire is the second most widely used synthetic monocrystalline material next to silicon. Sapphire represents one crystalline form of aluminum oxide (Al03) and may be formed by growing crystalline boules of sapphire from a melt, for example. Applications for sapphire include use as light emitting diode (LED) substrates, optical windows, silicon-on-sapphire (SOS), mobile devices, etc. All these applications entail single crystal (monocrystalline) sapphire in the form of thin crystalline sheets. Accordingly, when grown as boules, sapphire sheets or wafers may be formed by slicing the boules after growth to form a sapphire sheet or substrate of a target thickness.
In the case of silicon growth, monocrystalline silicon may be grown by a so-called floating silicon method (FSM). In the FSM method crystalline sheets of silicon are formed from a silicon melt by cooling a portion of the melt surface to crystallize a layer at the melt surface, and by pulling the crystalline layer in a horizontal direction. In this manner, a sheet of monocrystalline silicon may be continuously drawn as a ribbon from the silicon melt. Fortuitously, the density of monocrystalline silicon is less than the density of the silicon melt, causing the growing silicon ribbon to float on the melt surface. This allows the ribbon to be conveniently drawn along the melt surface and separated from the silicon melt at a target location. Other materials systems where monocrystalline sheet formation from a melt may be desirable, including sapphire, exhibit higher density in the crystalline phase than in the liquid phase. Accordingly, techniques for forming sapphire by drawing a horizontally oriented sheet from a melt are lacking. The present disclosure is provided in view of these considerations and other considerations.